XMind brainstorming software supports export to all kind of formats, including Microsoft PowerPoint presentations or slideshows (.ppt, .pps, .pptx, .ppsx). However, not all export options are available in the free version of the program and some may need a commercial license.
Some viable alternative would be to find another brainstorming tool that supports Xmind formats and also allows export and saving to .pptx files.